In Third Generation Partnership Project (3GPP) Technical Specification (TS) 38.300 V0.4.1, the New Radio (NR) handover mechanism is described as below.
Network controlled mobility applies to User Equipment devices (UEs) in RRC_CONNECTED and is categorized into two types of mobility: cell level mobility and beam level mobility. A beam when used herein is a reference signal transmitted within a cell i.e., a cell coverage may be obtained based on the combination of different reference signals termed as beams.
Cell Level Mobility requires explicit Radio Resource Control (RRC) signalling to be triggered, i.e. handover. For inter-NR base station (gNB) handover from a source gNB to a target gNB, the signalling procedures comprise at least the following elemental components illustrated in FIG. 1:
The source gNB initiates handover and issues 100 a handover Request to the target gNB over an Xn interface. The Xn interface is an interface between gNBs.
The target gNB performs 102 admission control and provides 104 an RRC configuration as part of the handover Acknowledgement. Admission control is a validation process in communication systems where a check is performed before the permission is granted for a connection to be established so that the available resources at the gNB are sufficient for the proposed connection. The RRC configuration comprises of parameters required for accessing the gNB, configuration of user and control plane to be used in the gNB.
The source gNB forwards 106 the RRC configuration to the UE in the handover Command. The handover Command message includes at least cell Identity (ID) and the information required to access the target cell so that the UE may access the target cell without reading system information broadcasted by the target cell. For some cases, the information required for contention-based and contention-free random access may be included in the handover Command message. The access information to the target cell may include beam specific information, if any.
The UE switches 108 to a new cell by moving the RRC connection to the target gNB and replies 110 the handover Complete.
In addition, in RAN2 #97bis meeting, there are the following further agreements for the procedures between steps 3 and 4:
Agreements1Handover command can contain at least cell identity of the targetcell and Random Access Channel (RACH) configuration(s) associatedto the beams of the target cell. RACH configuration(s) caninclude configuration for contention-free random access.1bUE selects a suitable beam from all beams of the targetcell.1cUE performs CBRA on the UE's selected beam if CFRAresources are not provided for the UE's selected beam.